The invention relates to a cleaning apparatus for removing solidified smelt accumulations that block or restrict the discharge of smelt from a chemical recovery combustion chamber. More particularly, the invention relates to a cleaning apparatus and a smelt discharge assembly for removing solidified smelt accumulations from a smelt spout and from a combustion device outlet port.
Wood pulp for paper making is usually manufactured by a sulfate process, where wood chips are cooked in a cooking liquor (typically known as “white liquor”) containing sodium sulfide and sodium hydroxide. After cooking, the used liquor (typically known as “black liquor”) is washed out of the pulp and treated in a recovery unit where the cooking chemicals are refined. Without reclamation and reuse of the cooking chemicals, the cost of the paper-making process would be prohibitive.
The recovery unit typically includes boiler tubes extending along the interior of the boiler walls. Concurrently with the reclamation process, the heat from combustion process is utilized to generate process steam within the boiler tubes for generating electricity and/or for other applications.
During the recovery process, the black liquor is first concentrated by evaporation into a solution containing approximately 65 to 80 percent solids and the solution is sprayed into the internal volume of a chemical reduction furnace. Inside of the chemical reduction furnace, the organic materials in the black liquor are combusted by various processes such as evaporation, gasification, pyrolysis, oxidation, and reduction, thereby reducing the black liquor into a molten smelt of spent cooking chemicals. The molten smelt exits the furnace through a boiler outlet port and flows along a smelt spout to a collection tank. The boiler outlet port and the smelt spout are designed to drain the molten smelt from the internal volume of the furnace at a desired rate in order to maintain a safe smelt level within the furnace and in order to maximize the efficiency of the furnace, as will be discussed in more detail below.
The molten smelt exits the boiler at a temperature of approximately 1000 degrees Celsius and, upon contact with ambient air, a top layer of the smelt may cool enough to become hardened and form hardened deposits and/or a hardened crust on top of the molten smelt in the outlet opening and/or spout. Hardened smelt may obstruct the flow of the molten smelt, thereby reducing the effectiveness of the outlet port and smelt spout and causing the smelt level within the furnace to be undesirably high. Additionally, a reduced smelt flow may cause the molten smelt to remain in the smelt spout longer, thereby increasing the time that the smelt is subject to ambient temperatures and increasing the likelihood that more hardened deposits will form. Therefore, the hardened deposits may tend to form within the smelt spout at a rapid rate.
A high smelt level can cause a wide range of problems or undesirably low production levels. For example, a high smelt level may cause inefficient and unpredictable furnace operation, such as: a decrease in the amount of chemicals that can be recovered; a decrease in the process steam outputted from the boiler tubes; an increased emission of noxious gases such as carbon monoxide and sulfur dioxide. As another example, the hardened smelt may cause the molten smelt to splash out of the spout, thereby causing dangerous conditions for nearby workers and/or potentially causing property damage. Moreover, the smelt can build up to a dangerous level and either block furnace air ports, potentially causing the fire to be extinguished, or fill up the furnace windbox, causing serious corrosion problems or even causing smelt to pour out onto the floor adjacent the furnace. As yet another example, a high smelt level may cause a rapidly increase in temperature which may lead to a boiler explosion.
Typically, hardened deposits are manually dislodged from the outlet port and the spout at regular intervals. For example, workers hold a long rod with a tool attached to the distal end so as to scrape hardened deposits from the spout and/or outlet port. However, such manual rodding of the smelt spout and outlet port is inefficient, unsafe, and is a tedious, physically demanding job that may fatigue operators. Additionally, smelt spouts are cooled by water circulating in a water jacket surrounding the spout, which can become ruptured by improper rodding. A broken water jacket can result in an explosion in the furnace. Other dangers to workers include the potentially hazardous fumes from the collection tank. Furthermore, the regular intervals at which the hardened smelt must be removed causes labor costs to be undesirably high.
Recently, automated devices have been used to automatically, periodically scrape hardened deposits from the spout and/or outlet port. For example, U.S. Pat. No. 4,706,324, which issued Nov. 17, 1987, discloses a smelt spout cleaner that is mounted on or above the smelt spout. A housing is mounted above the smelt spout and, at regular intervals, a cleaning head assembly swings in a downward, sweeping stroke from the housing towards the spout to clean deposits from the boiler outlet port and then swings in an upward, sweeping stroke toward the housing so as to mirror the downward stroke and to clean deposits from the spout. The cleaning head assembly includes a cleaning head that enters the boiler outlet port on the downward stroke. Additionally, the cleaning head assembly includes pivotable channel scraping members that each has a shape and size that generally matches that of the spout. During the downward stroke, the channel scraping members each pivot into a collapsed state to ride on the top of the molten smelt flow rather than entering the flow. Then, during the upward stroke, the channel scraping members pivot back into an extended state and are scraped along the side and bottom walls of the spout.
However, because the width of each of the channel scraping members is generally equal to the width of the spout, the flow of molten smelt is disrupted by the scraping members during the upward stroke, thereby potentially causing the molten smelt to splash or overflow from the spout. Additionally, although the hardened smelt deposits generally only occur at the top layer of the smelt flow, the channel scraping members in the '324 patent each scrape along the bottom walls of the smelt spout, thereby exposing the entire spout to potential premature wear when only select portions of the spout require regular cleaning. Furthermore, the design disclosed in the '324 patent only cleans the spout along arcuate cleaning paths traveled by the scraping members so that portions of the spout that lie between the cleaning paths may remain uncleaned. Conversely, if additional scraping members are added to the design disclosed in the '324 patent to minimize gaps between the cleaning paths, then the spout may be subject to unnecessary part wear. Additionally, the upward cleaning stroke lifts the hardened smelt deposits upwards and out of the smelt spout, increasing the possibility of smelt splash and/or overflow.
Another automated device for scraping hardened deposits from the spout and outlet port is disclosed in U.S. Pat. No. 5,542,650, which issued on Aug. 16, 1996. The '650 patent discloses a cleaning head assembly that travels along a smelt spout in a direction generally parallel thereto to scrape hardened deposits from the spout walls. More specifically, the cleaning head assembly includes a plurality of U-shaped paddles that have a size and shape corresponding to that of the smelt spout so that the paddles fit within the spout and dislodge hardened deposits from the surfaces thereof as they are moved along a substantial portion of the length of the spout.
However, because the width of each of the paddles is generally equal to the width of the spout, the flow of molten smelt is disrupted by the paddles, thereby potentially causing the molten smelt to splash or overflow from the spout. Additionally, although the hardened smelt deposits generally only occur at the top layer of the smelt flow, the paddles each scrape along the bottom walls of the smelt spout, thereby exposing the entire spout to potential premature wear when only select portions of the spout require regular cleaning. Furthermore, because the cleaning head assembly is translated along a substantial length of the smelt spout during cleaning, the cleaning cycle may take an undesirable amount of time to complete.
As seen from above, it is desirous to provide an improved smelt spout assembly and a cleaning apparatus for cleaning a smelt spout to improve the efficiency and effectiveness with which a smelt spout and/or a boiler outlet port can be cleaned.